The present invention relates generally to an improved femoral stem for use in hip joint arthroplasty. More specifically, the present invention relates to a femoral stem with a partially recessed porous coating that effectively forms a smooth arcuate transition at least between sections of the porous coating and the stem body to provide for smooth insertion of the femoral stem into a bone cavity during surgery, while maximizing press-fit engagement within the bone cavity.
In hip arthroplasty, a broach instrument is used to cut out a cavity in the proximal femur for seated reception of a femoral stem. Preferably, this cavity provides for at least some press-fit engagement of the porous coating of the femoral stem such that the bone surrounding the cavity is encouraged to grow into the porous coating over time. As a result, the femoral stem better locks into the cavity to secure the implant to the bone. To achieve this proximal press-fit engagement, femoral stems known in the art include a proximal porous coating that protrudes beyond the distal femoral stem body. In this respect, FIG. 1 illustrates a set of three such blade/Mueller type femoral stems 20, 20′, 20″ known in the prior art having an overhang or step 22, 22′, 22″ formed by application of a respective porous coating 24, 24′, 24″ to a respective portion of the stem body 26, 26′, 26″. FIG. 2 is an enlarged view of the prior art femoral stem 20 of FIG. 1, taken about the circle 2, and more specifically illustrates the outwardly protruding proximal porous coating 24 that creates the step 22 along the stem body 26. This ledge or step 22 is particularly undesirable on the medial side of the femoral stem 20 as it can cause the femoral stem 20 to hang up while being seated into the femoral bone cavity during surgery. If the femoral stem 20 does hang up during surgery, it may lead to difficulties seating the femoral stem 20, poor interdigitation of the bone with the porous coating 24, intraoperative and/or postoperative fracture, and/or poor fixation in the femoral cavity over the long-term.
Some femur stem designs known in the art attempt to rectify issues related to hang up by forming a recessed area around the entire perimeter of the femoral stem where the porous coating is applied. Example prior art devices are shown and described in U.S. Pat. Nos. 4,778,475; 5,013,324; 4,828,566; 4,608,055; 4,938,772, the contents of which are herein incorporated by reference in their entirety. The benefit of such a recessed area is that a porous coating having a certain thickness approximately equal to the depth of the recess can be applied to the stem body so the area of the porous coating is generally flush with the surrounding stem body, effectively eliminating the step. But, the drawback is that seated insertion of the femoral stem into the femoral cavity provides insufficient surface area contact of the porous coating with the bone because the porous coating is no longer outwardly extending. The Zimmer APS Natural-Hip System is another product currently available on the market that includes a recessed section on the stem body for receiving the porous coating. But, the Zimmmer APS Natural-Hip System only uses such a recessed area as a bone ingrowth surface, and not to facilitate insertion of the femur stem into the cavity of the femur. Thus, prior art devices either include the ledge or step 22, 22′, 22″ (thereby prone to hang up) and provide desired engagement between the porous coating and the bone, or the prior art femur stems have a recessed surface for reception of the porous coating, such as the Zimmer Natural Hip, and are undesirably incapable of press fitting the porous coating into the femur cavity to achieve desired engagement of the femur stem with the femur over the long term. Consequently, the prior art fails to disclose a recessed area or pocket along less than the entire perimeter of the femoral stem implant (e.g., only the medial side of the femoral stem implant), to eliminate the step, while maintaining substantial surface area engagement of the porous coating within the femur cavity.
To more specifically illustrate the abovementioned drawback, FIGS. 3-8 illustrate insertion of the prior art femoral stem 20 into a mockup of a femoral bone cavity 28 after broaching. More specifically, FIG. 3 illustrates initial insertion of the femoral stem 20 into the bone cavity 28. As shown in the enlarged view of FIG. 4, the stem body 26 easily fits into the enlarged upper end of the bone cavity 28 for slide-in reception. The hang up is not particularly problematic at this stage given that the step 22 is still well above an upper rim 30 of the bone cavity 28 formed from broaching. But, as the femoral stem 20 is continually inserted into the bone cavity 28, the medially outwardly projecting step 22 may have a tendency to catch on the upper rim 30, especially if the femoral stem 20 is inserted deeper into the bone cavity along its medial edge. In this respect, the surgeon must ensure that the femoral stem 20 is positioned somewhat away from the upper rim 30 so the medially outwardly projecting step 22 does not catch or hang up on this rim 30. If it does, the step 22 may catch, chip, fracture, or otherwise damage the bone of the upper rim 30, which can be particularly problematic when the femoral stem 20 is hammered into place. Moreover, required offsetting to clear the step 22 from the upper rim 30 may cause the femoral stem 20 to be misaligned when hammered into place, as previously mentioned above.
Even if the surgeon successfully clears the upper rim 30 without causing damage thereto, an undesirably low surface area of the porous coating 24 may abut or sit adjacent to the bone cavity 28. In this respect, FIG. 5 illustrates continued insertion of the femoral stem 20 into the bone cavity 28 as the stem body 28, and particularly as a distal tip 32, nears the bottom of the bone cavity 28. Here, the step 22 has already bypassed the upper rim 30, as better shown in FIG. 6. The femoral stem 20 is shown somewhat offset from the inner surface of the bone cavity 28, which provides for somewhat of a gap 34, as shown. When the femoral stem 20 is finally seated as shown in FIG. 7 and more specifically in the enlarged view of FIG. 8, the step 22 has a general single point of contact with the inner wall of the bone cavity 28, which forms the offset or gap 34 between the stem body 26 and the inner wall of the bone cavity 28. This gap 34 is particularly undesirable as it decreases the surface area contact of the porous coating 24 with the bone cavity 28 for purposes of ingrowth engagement after the formal stem 20 is implanted.
There exists, therefore, a significant need in the art for a femoral stem with a medially recessed porous coating limited only to the distal medial side of the coated area, which facilitates easier stem insertion by reducing the prior art step at the distal medial edge of the porous coating, while still allowing the femoral stem to simultaneously press-fit engage the entire porous coated surface to the femur cavity, except at the distal medial corner where there is no step. The present invention fulfills these needs and provides further related advantages.